Merck KGaA said U.S. drug regulators' concerns about the risks of its cladribine pill will put an end to any development or marketing plans for the multiple sclerosis (MS) treatment. Merck also said on Wednesday it planned to withdraw cladribine from markets in Australia and Russia where it has been approved and is available under the name Movectro. Its shares were down 1.8 percent, underperforming a 0.5 percent lower European healthcare sector index . "Merck believes that data from ongoing clinical trials are very unlikely to address the (U.S. Food and Drug Administration's) requirements," it said, adding new trials would not justify the costs.